1. Field of the Invention
The invention is generally related to photoresist compositions used in the fabrication of microelectronic circuitry and, more particularly, to a novolak photoresist composition including a photosensitizer and a polyalkylmethacrylate co-polymer of polyhydroxystyrene.
2. Description of the Prior Art
Novolaks are thermoplastic phenol-formaldehyde type resins that are obtained primarily by the use of acid catalysts and excess phenol. These types of resins are generally alcohol and alkali soluble, and can be reacted to form insoluble, crosslinked materials. Novolaks have been used extensively in the microelectronics industry as photoresist materials that are used for patterning substrates such as semiconductor wafers and chips, as well as printed circuit boards and other substruates. For example, U.S. Pat. No. 4,237,216 to Skarvinko discloses a photosensitive patternable coating composition containing novolak materials, U.S. Pat. No. 4,920,028 to Lazarus et al. discloses a high contrast positive photoresist which includes a novolak prepared with mixed cresol and hydroxybenzaldehyde together with a diazoquinone sensitizer, and U.S. Pat. No. 4,980,264 to Chiong et al. shows a photoresist composition that has a controlled dissolution rate in alkaline developers, and each of these patents are herein incorporated by reference.
Because the synthesis of novolak involves condensation reactions which produces linear and branched structures of wide polydispersity and inconsistent physical properties, it is common practice to blend various molecular weight grades to obtain more consistent performance (see, T. R. Pamapolone, Solid State Technology, 6/1984, pgs 115-120). This procedure adds extra cost and time to adjust photospeed by the addition of novolak resin blend partners and photoactive components.
In patterning operations, a photoresist composition is deposited on a substrate, typically as a layer on the order of several microns thick, and portions of the layer are selectively exposed to energy (UV, I-line, infrared, X-rays, etc.). The exposure step is most often performed using a mask with the photoresist being exposed at only the portions not covered by the mask. The exposure step causes a chemical change in the exposed regions relative to the non-exposed regions. For "positive" photoresists, the exposed region becomes soluble, while the unexposed regions remain insoluble. The exposed regions are then removed and the hardened and patterned photoresist remains in place on the substrate for further processing operations (i.e., reactive ion etching (RIE), chemical vapor deposition (CVD), etc.). For "negative" photoresists, the exposed region becomes insoluble and may be subject to subsequent processing (e.g., heat treatment, etc.). Thus, the unexposed regions remain soluble in a negative photoresist and the exposed regions of the photoresist remain for subsequent processing steps to be performed.
In traditional photoresists which employ novolaks, such as cresol resins and related monomers such as resorcinol, ethyl phenol, xylenols, etc., and photoactive compounds such as diazonaphthoquinones (DNQs), the resist photospeed and performance have usually been optimized by blending a low molecular weight resin ("fast" resin) and a high molecular weight resin ("slow" resin) . For example, U.S. Pat. No. 5,374,693 to Lynch et al., which is herein incorporated by reference, discloses using a mixture of novolak resins which have similar dissolution rates, but which have dissimilar molecular weights, as a photoresist material for patterning semiconductor substrates. Lynch et al. particularly recommends using novolaks with molecular weights that differ by more than 10-60%.
While blending of novolaks is widely used in the electronics industry, there are several disadvantanges. The most significant disadvantage is that because novolaks are highly branched polymers, there is a wide variation in lot-to-lot performance of the novolak resin mixtures, especially when using low molecular weight or "fast" novolaks.
One approach for enhancing the performance of novolaks in DNQ resists which has been attempted is to use poly(p-hydroxystyrene) as a blending partner. When poly-p-hydroxystyrene (PHS) has been used as a blending partner with novolak and diazoquinone inhibitor (see, Hattori et al., Jap. J. Appl. Phys., 30, 3125 (1991)), it was found that blended partners with high or low molecular weight PHS exhibit a weak dissolution inhibition effect.
The observations of Hattori et al. have been confirmed by the inventors of the present patent application. Specifically, they have observed loss of the unexposed resist with PHS-novolak DQ blends, and surface pitting occurred, presumably due to the loss of PHS from the matrix.
Other constituents have been added to novolak photoresists to achieve particular results. For example, U.S. Pat. No. 5,275,921 to Koizumi discloses included Sudan Orange dye in the photoresist material to decrease light reflection from the substrate, U.S. Pat. No. 5,256,521 to Jayarama discloses adding a tris(hydroxyphenyl) lower alkane to a photoresist composition as a sensitivity enhancer, and U.S. Pat. No. 5,275,911 to Toukhy discloses the addition of sesamol/aldehyde condensation products in the photoresist material as sensitivity enhancers, and each of these patents are herein incorporated by reference.